1. Field of the Invention
This invention relates to scrim-reinforced plastic film and a process for the preparation thereof and particularly to such film having improved strength properties, the film being advantageously adapted for use in preparing various types of containers, such as envelopes, bags etc.
2. Description of the Prior Art
Methods for the preparaton of scrim-reinforced plastic sheet are well known in the art. Since plastic materials currently available commercially lend themselves advantageously to a wide variety of uses and particularly in the packaging of consumer goods, there has been an ever-increasing emphasis on developing means to further enhance the strength properties of such materials. Thus, tensile strength, tear and impact resistance, moistureproofing and the like are especially important properties as regards container utility. The use of scrims, these usually comprising web material formed of spaced-apart strands of high strength thermoplastic resinous material, to reinforce plastic sheets has proved particularly effective in augmenting tensile strength, tear resistance and the like. One method commonly used for the preparation of scrim-reinforced plastic sheet involves sandwiching a scrim member between two sheets of thermoplastic material the assembly thereafter being heat bonded by various means, e.g., contacting opposed sides of the sandwich assembly with heated platens, radiant heating etc.
Thus, one such method involves the radiant heating of the edge portions of plastic material folded back on a scrim member to form an envelope portion. Other methods involve, for example, the application of adhesive to the scrim or web and/or the inner surfaces of the plastic sandwiching elements and pressure-sealing the elements together; extruding one or more layers of thermoplastic material onto a supported scrim or web i.e., provided with a thermoplastic backing layer.
Methods of the aforedescribed type entail significant disadvantages. Thus, the use of heated rollers invariably results in an undesirably thin enclosing plastic film; moreover, the combined heat-pressure effects often causes structural weakening of the film and particularly in the critical areas situated outside of the strands. Extrusion of plastic onto a supported web or scrim requires the use of a plastic backing or supporting sheet. Thus, there may be significant differences in the strength characteristics as between the opposed plastic members sandwiching the web. Bonding can occur between the outer plastic elements, or between each of these and the intermediate web which can produce sub-optimum strength characteristics in the product.
The foregoing methods require formation of a sandwich type element wherein the outer plastic layers are separately provided. This requires rather burdensome techniques for carrying out continuous operations. This aspect combined with the sub-optimum strength characteristics often obtained in the final product underscore the need for more effective methods for preparing reinforced plastic sheets.
Thus, a primary object of the invention is to provide a method for preparing reinforced plastic sheets wherein the foregoing and related disadvantages are eliminated or at least mitigated to a substantial extent.
Another object of the present invention is to provide such a method particularly advantageously adapted to continuous processing.
Yet another object of the invention is to provide such a method wherein any necessity for the use of heated pressure members such as platens to effect the bonding operation is eliminated.
Still another object of the invention is to provide such a method advantageously adapted to be carried out in conjunction with an extruder.
A further object of the invention is to provide such a method capable of producing a reinforced plastic sheet having improved strength properties including, tensile strength, tear resistance, impact resistance and the like.
A still further object of the invention is to provide such a method which can readily be integrated with a process for producing plastic containers such as envelopes, containers, etc.
Yet a still further object of the invention is to provide a reinforced plastic sheet material having significantly improved strength characteristics.
A still further object of the invention is to provide a scrim reinforced plastic sheet wherein the scrim reinforcement is apparent both visually and tactilely.
Other objects and advantages of the invention will become more apparent hereinafter as the description proceeds.